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I suspect the usable number is much lower and would be just those satellites mostly opposite the Earth but with some signal reaching the moon? I recall the beam width of GPS antennas being like 30 deg (?), so almost all of the signal is directed at Earth.

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They'll be using sidelobes. This is what's done at geo altitude which is also above the GPS orbits.

At geo the commercial satellite I worked with had position accuracy within about ten meters, and we always had access to 6-8 GPS satellites at a time. Obviously at the moon the signal is much fainter but my understanding is that it's essentially the same just harder to detect

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